SimplifyGlobals: Optimize away globals that are only read in order to write themselves (#4070)

If the only uses of a global are

if (global == 0) {
  global = 1;
}

Then we do not need that global: while it has both reads and
writes, the value in the global does not cause anything observable.
It is read, but only to write to itself, and nothing else.

This happens in real-world code from j2cl quite a lot, as they have
an initialization pattern with globals, and in some cases we can
optimize away the work done in the initialization, leaving only the
globals in this pattern.

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